This invention relates to pretreating a metal with a composite layer containing siloxane for forming an adherent covalent bond between an outer paint layer and the metal substrate. More particularly, the invention relates to a one-step process for pretreating metal with an alkaline solution containing at least one of a dissolved inorganic silicate and a dissolved inorganic aluminate, an organofunctional silane and a non-functional silane crosslinking agent.
It is known to improve corrosion resistance of cold rolled and metallic coated steels by passivating the surface with a chromate coating. Because of the toxic nature of hexavalent chromium, rinses containing chromate ions are undesirable for industrial usage.
It also is known to treat cold rolled and metallic coated steels with a phosphate conversion coating to improve paint adherence. To improve the corrosion performance, however, these phosphated steels generally require chromate final rinse.
It has been proposed to improve corrosion resistance and paint adhesion on cold rolled and galvanized steel by coating with an inorganic silicate and then treating the silica coating with an organofunctional silane. U.S. Pat. No. 5,108,793 discloses forming the silica coating by rinsing the steel with an alkaline solution containing dissolved silicate and metal salt. The steel is dried to form a silica coating having a thickness of at least 20 .ANG.. Thereafter, the silica coated steel is rinsed with an aqueous solution containing 0.5-5 vol. % organofunctional silane. The silane forms a relatively adherent covalent bond between the silicate coating and an outer paint layer.
There have been numerous other proposals to improve corrosion resistance and paint adhesion on cold rolled and galvanized steels. Some artisans have proposed pretreating the steel with a chromate solution containing colloidal silicate and/or aluminate and silane. Others have proposed rinsing the steel with a chromate solution and then rinsing the chromated steel with a solution containing colloidal silicate or aluminate and silane. Still others have proposed rinsing the steel with a solution containing polymeric resin, colloidal silicate and silane.
As evidenced by the effort of previous workers, there has been a long felt need to develop a process for improving corrosion resistance of and paint adherence to a metal using environmentally safe coating solutions that can be disposed of inexpensively. The process should be low cost, use nontoxic materials that can be safely disposed of, provide long term resistance in a humid environment and not require complex multiple step processing or chromating.